This invention relates to centrifugal vapor compressors, and more particularly to controlling vapor flow through a diffuser passage of a centrifugal vapor compressor.
One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates.
One of the most accepted and successful modifications has been the addition of guide vanes in the inlet of the compressor to vary the flow direction and quantity of entering vapor. Another widely known modification has been to vary diffuser configuration in response to the load on the compressor. Commonly, this is done by means of a diffuser control ring which moves laterally across the diffuser passage to throttle vapor flow therethrough. Prior art variable diffuser control rings have been generally controlled by a mechanism arranged to locate and hold the control ring at any position between a full open and a full closed position. Such control mechanisms are typically relatively expensive, often involving fairly complex mechanical and/or pneumatic components. Further, because of the complex components, the manufacture and installation of diffuser ring control mechanisms are often difficult and time consuming tasks requiring expensive skilled manual labor. While continuously variable diffuser control rings often provide excellent results, it has been learned that very satisfactory results can be achieved with a diffuser control ring which has a limited number of discrete, spaced throttling positions. While obtaining these very satisfactory results, a discretely variable diffuser control, in accordance with the present invention, is, at the same time, much simpler than prior art diffuser controls. This simplicity facilitates and reduces the cost of construction, installation, and maintenance of the diffuser control and improves the reliability thereof.